1. Field of the Invention
The present invention relates to a fixing device to be mounted to an image forming apparatus, such as an electrophotographic printer, facsimile machine, or copying machine. More particularly, the present invention relates to a fixing device utilizing an electromagnetic induction heating technique as a fixing technique.
2. Description of the Related Art
A conventional image forming apparatus that uses toner for forming a visible image includes a fixing device that fixes a toner image onto a recording material. Such a fixing device comprises a fixing roller (or a heating roller) for heating and melting a non-fixed toner image, and a pressure roller that presses a recording material against the fixing roller so as to sandwich and transport the recording material with the fixing roller. Conventionally, such a fixing device comprises a halogen lamp as a heater inside the fixing roller. Such a halogen lamp heats the fixing roller to a temperature high enough for toner image fixation.
However, the above heating method utilizing a heater requires a relatively long time to heat the fixing roller to a predetermined temperature, and during the heating, a user cannot use the image forming apparatus and is forced to wait for a long time. Also, since a halogen lamp heater has great heat loss, the energy consumption becomes large in the halogen lamp heating method. Such a problem cannot be overlooked in this time and age of environmental-friendly movement, and there has been an increasing demand for an efficient fixing device with a short rise time.
In view of the above facts, an induction heating fixing device that heats a fixing roller made of a metallic conductor by eddy current caused by electromagnetic wave has been drawing attention, because such a fixing device dramatically shortens the heating time.
FIG. 1 shows the structure of a conventional induction heating fixing device. In this device, an induction coil 1 is inserted into a heating roller 3 that is in pressure-contact with a pressure roller 2. The induction coil 1 is wound around a bobbin 4, so that high-frequency current flows through the induction coil 1 in its axis direction via a lead wire attached at either end. The induction coil 1 is held by brackets attached to side plates (not shown), and thus prevented from rotating. In FIG. 1, the heating roller 3 rotates clockwise, while the pressure roller 2 rotates counterclockwise. A temperature fuse 5 and a thermistor 6 are disposed on the opposite side of the surface of the heating roller 3 from the pressure roller 2.
During the fixing operation, high-frequency current flows through the induction coil 1 via the lead wires, generating a high-frequency magnetic field. This high-frequency magnetic field generates induction eddy current in the heating roller 3 made of a metallic conductor, and the fixing roller generates Joule heat due to skin resistance. As shown in FIG. 1, a recording sheet S having a toner image T1 to be fixed thereon is transported from the right in the direction of the arrow. The recording sheet S is then sandwiched by the heating roller 3 and the pressure roller 2, so that the toner image T1 is fixed by heat and pressure.
In the above structure, a heating coil for induction-heating the heating roller is inserted into the heating roller so as not to hinder the sheet transportation. Here, the heating roller generally has a cylindrical shape, and the inside of the core bar is not processed. A separation layer is formed outside the core bar, but the separation layer is not directly attached to the core bar, because it is normally difficult for a separation layer to adhere to the surface of the core bar. Therefore, a primer layer is employed to help the bonding between the core bar and t he separation layer.